Put mushroom stems in a pot with 4 cups water. Bring to a boil; reduce heat; simmer 25 to 30 minutes. Strain and return liquid to pot. Reduce liquid by half to make about 2 cups of mushroom stock.

Saute portobello mushrooms in heated olive oil for 4 to 5 minutes. Using a slotted spoon, transfer mushrooms to a bowl. Add celery and onion to oil and wilt down for 4 to 5 minutes. Add garlic and cook 1 minute. Then add flour to make a light roux.